The long term goal of this research is to elucidate the mechanism by which LH trophically regulates the number, ultrastructure, and steroidogenic function of Leydig cells in adult rat testes. We will use stereological techniques (quantification of 3-dimensional structures from 2-dimensional cross-sections) to determine the number, and volume of Leydig cells and the surface area of Leydig cell cytoplasmic organelles which contain steroidogenic enzymes. Also, we will use in vitro testicular perfusion to determine testosterone secreting capacity, mitochondrial cholesterol side chain cleavage (Cscc) activity, and smooth endoplasmic reticulum (SER) 17 alpha Hydroxylase/C17, 20 Lyase activity in Leydig cells in which the cytoarchitecture and intrinsic physiologic controls are intact. Finally, we will use a specific antibody to determine the biosynthesis and degradation of 17 alpha Hydroxylase/C17,20 Lyase; the rate limiting steroidogenic reaction in the Leydig cell SER enzyme sequence responsible for the conversion of pregnenolone to testosterone. Information will be forthcoming on: whether LH trophically regulates Leydig cell division and/or differentiation; whether LH controls Leydig cell mitochondria growth and/or division and Cscc activity; and whether LH regulates 17 alpha Hydroxylase/C17,20 Lyase biosynthesis, activity and/or degradation.